Targeting mTOR/YY1 signaling pathway by quercetin through CYP7A1-mediated cholesterol-to-bile acids conversion alleviated type 2 diabetes mellitus induced hepatic lipid accumulation

Abstract

Background: Hepatic lipid accumulation was a major promoter for the further development of non-alcoholic fatty liver disease (NAFLD) in type 2 diabetes (T2DM). mTOR/YY1 signaling pathway regulated many metabolic processes in different organs, and played an important role in hepatic lipid metabolism. Thus, targeting mTOR/YY1 signaling pathway might be a novel therapeutic strategy of T2DM-associated NALFD.

Purpose: To investigate the effects and the mechanism of quercetin against T2DM-associated NAFLD.

Study design and methods: The combine abilities of 24 flavonoid compounds with mTOR were detected by computer virtual screening (VS) and molecular modeling. mTOR/YY1 signaling pathway was examined in the liver of db/db mice, and high glucose (HG) and free fatty acid (FFA) co-cultured HepG2 cells. YY1 overexpression lentivirus vector and mTOR specific inhibitor rapamycin were used to further identify the indispensable role of mTOR/YY1 signaling pathway in quercetin's amelioration effect of hepatic lipid accumulation in vitro. Clinical studies, luciferase assay and chromatin immunoprecipitation (ChIP) assay were all carried out to investigate the potential mechanisms by which quercetin exerted its amelioration effect of hepatic lipid accumulation.

Results: Quercetin had the strongest ability to combine with mTOR and could competitively occupy its binding pocked. Along with the alleviated hepatic injury by quercetin, mTOR/YY1 signaling pathway was down-regulated in vivo and in vitro. However, the alleviation effect of quercetin against hepatic lipid accumulation was inhibited by YY1 overexpression in vitro. Mechanistically, the down-regulated nuclear YY1 induced by quercetin directly bound to CYP7A1 promoter and activated its transcription, resulting in the restoration of cholesterol homeostasis via the conversion of cholesterol-to-bile acids (BAs).

Conclusion: The hepatoprotective effect of quercetin on T2DM-associated NAFLD was linked to the restoration of cholesterol homeostasis by the conversion of cholesterol-to-BAs via down-regulating mTOR/YY1 signaling pathway, leading to the increased CYP7A1 activity.

Keywords: Bile acids; CYP7A1; Cholesterol homeostasis; Quercetin; T2DM-associated fatty liver disease; mTOR/YY1 signaling pathway.